<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .hello{
            width: 200px;
            height: 200px;
            background: #ccc;

            /* border */
            /* border-top: 1px solid red;
            border-right: 5px dashed blue;
            border-bottom: 3px dotted green;
            border-left: 2px solid red; */
            border: 2px solid red;

            /* padding */
            /* padding-left: 10px;
            padding-top: 20px; */
            padding: 20px;
            /* padding: 20px 30px; */
            /* padding: 10px 20px 30px 40px; */

            /* margin */
            /* margin-bottom: 50px; */
            /* margin-left: 400px;
            margin-top: 100px; */

            /* 盒子/容器的水平居中 */
            margin: 0 auto;

            text-align: center;
            line-height: 200px;

        }
        .world{
            width: 200px;
            height: 200px;
            background: #666;

            /* margin-top: 50px; */
        }
    </style>
</head>
<body>
    <div class="hello">
        嘿嘿
    </div>
    <div class="world">
        哈哈
    </div>
</body>
</html>